绑定对象不在FXML UI控制器initialize（）方法之外工作

Question

核心问题：如果绑定属性在声明绑定属性的initialize()方法之外更新，则binding对象无效侦听器不会触发。

以JavaFX UI控制器类中声明的initialize()方法为例：

@FXML
private void initialize() {
    final StringProperty stringProperty = textField.textProperty();
    stringProperty.addListener(
            (observable, oldValue, newValue) -> System.out.println("stringProperty value: " + newValue));
    Bindings.createStringBinding(() -> "PREFIX - " + stringProperty.getValue(), stringProperty).addListener(
            (observable, oldValue, newValue) -> System.out.println("StringBinding value: " + newValue));

    // Editing stringProperty value inside initialize() method
    stringProperty.setValue("u");
    stringProperty.setValue("ua");
    stringProperty.setValue("ua");
    stringProperty.setValue("uaa");
}

如您所见，我声明了一个StringBinding，它依赖于TextField的text属性，称为StringProperty，并声明了一个ChangeListener，它在StringBinding无效时请求计算。

如果我在initialize方法中编辑StringProperty值，则会触发StringProperty和StringBinding更改侦听器，而如果我从UI编辑StringProperty值，则只会触发StringBinding更改侦听器。

有人能解释一下为什么会这样吗？

Answer 1

由于不存在对bindings.createStringBinding创建的StringBinding的强引用，因此它最终将被垃圾回收。一旦发生这种情况，您添加的侦听器将与它一起被垃圾回收。

我不认为这是重点，因为绑定对象通过InvalidationListener侦听它们的依赖项(Observable对象），并且Obsevable.AddListener(InvalidationListener)文档指出，“Observable存储对侦听器的强引用，这将防止侦听器被垃圾收集，并可能导致内存泄漏。”

这是正确的，但请看XXXBinding类使用的侦听器实现：

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package com.sun.javafx.binding;

import java.lang.ref.WeakReference;

import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.beans.WeakListener;
import javafx.beans.binding.Binding;

public class BindingHelperObserver implements InvalidationListener, WeakListener {

    private final WeakReference<Binding<?>> ref;

    public BindingHelperObserver(Binding<?> binding) {
        if (binding == null) {
            throw new NullPointerException("Binding has to be specified.");
        }
        ref = new WeakReference<Binding<?>>(binding);
    }

    @Override
    public void invalidated(Observable observable) {
        final Binding<?> binding = ref.get();
        if (binding == null) {
            observable.removeListener(this);
        } else {
            binding.invalidate();
        }
    }

    @Override
    public boolean wasGarbageCollected() {
        return ref.get() == null;
    }
}

如您所见，添加到依赖项的侦听器实例（即可观察)是WeakListener，并且只维护对绑定的弱引用。这允许对绑定进行垃圾回收，即使它没有被正确释放。这样做是为了在绑定超出范围但可观察没有超出范围的情况下防止内存泄漏。

换句话说，Observable维护对InvalidationListener的强引用，但InvalidationListener维护对绑定的弱引用。

这种行为记录在“远程”位置，包括属性#bind(ObservableValue):

为此属性创建单向绑定。

注意，JavaFX的所有绑定调用都是通过弱监听器实现的。这意味着绑定属性可以被垃圾回收并停止更新。